1. Field of the Invention
The present invention relates to a method intended to reduce the exposure of an organism to radiation and infra-red, ultrasonic or magnetic pulse beams from medical imaging equipment, and comprises a method for treating a signal or a set of signals with a view to producing under greatly improved conditions digital images of the interior of an organism, in order to detect and treat where appropriate abnormalities or disorders in the examined organism.
2. Description of the Related Art
Various methods are now commonly used for this purpose, in particular radiography, scanography, echography and magnetic resonance imaging, in order to obtain and process information relating to a set of points or elementary zones located in an organism, so as to produce items of information relating to each point and capable of being reproduced in the form of images that can be used by medical practitioners to investigate abnormalities or disorders.
Conventionally a value, associated with each point or zone, of the coefficient of attenuation in the case of a medical scanner, of reflection in the case of echography, or of the proton density in the case of magnetic resonance imaging, is obtained by means of a suitable calculation in order to produce an internal image of the organism.
The example of the medical scanner illustrates the prior art in this respect.
Medical imaging is in fact a basic procedure for detecting and treating cancer and a certain number of serious conditions, or observing the internal organs of a patient. However, the medical community have felt it necessary to go much further in expressing the need for a high definition medical imaging method in order to achieve a significant improvement in obtaining and processing signals and images that they represent.
The principles of scanography and the current state of the art in this field should first of all be recalled.
Scanography (or tomodensitometry) was discovered in 1968 by G. N. Hounsfield, an engineer working for the EMI company.
The 1972 patent (U.S. Pat. No. 3,924,131, U.S. Pat. No. 3,919,552) is entitled “A method and apparatus for examination of a body by radiation such as X or gamma-radiation”.
In 1979 the inventor was awarded the Nobel prize for his invention.
The principle of the invention is as follows:
A beam of X-rays scans a defined plane, passes linearly through an organ, and strikes a plate or a radiographic detector. The passage through the organ produces an attenuation of the beam, the degree of attenuation being able to be measured by means of the detector. Crosswise scanning in the sectional plane produces a set of information that is processed by suitable software on an associated computer.
In fact, in a heterogeneous medium the attenuation along each scanning axis may be expressed by an exponential law, taking into account the photoelectric absorption and diffusion due to the Compton effect.
Let I0 be the reference value, Ix be the value at a point X, then one may write the following relationship:∫A(x)dx=F I=Ioe−F In=IoE 
From which one obtains by discretisation:
      L    ⁢                  ⁢    n    ⁢          Io      In        =                    ∫        0        t            ⁢                        A          ⁡                      (            x            )                          ⁢                  ⅆ          x                      =                            A          1                ⁢                  X          1                    +                        A          2                ⁢                  X          2                    +      …      +                        A          n                ⁢                  X          n                    
The successive values A1, A2 . . . , An correspond to the values of each segment defined by X1, X2 . . . , Xn.
The profiles of each scanning associated with a specific angle (or a specific position) may then be expressed by a series of equations.
A particular scale may be defined by the value relative to a reference value of the coefficient of attenuation, for example that of water or any other suitably chosen molecule.
The scale most often used is that relating to an abundant molecule in all living organisms, namely water.
If A (H2O) denotes the coefficient of attenuation of water, then a relative scale such as the following may be used:Bn=[An−A(H2O)]*1000/A(H2O)
The value of the coefficient of water may be defined as equal to 1 or 0, thereby creating a notation system that is easy to use since water is an essential component of the human body.
Other systems may however be used, according to the way in which the information obtained is expressed (visually). Often a value of 1000 is chosen for bone and a value of −1000 is chosen for air.
The information processing of a sufficient number of cross scannings, defining in fact small elementary cells or zones, enables a set of linear equations to be solved provided that the number of scannings is equal to the number of cells.
The editing and use of the information are carried out by an associated computer.
The computer collects the set of data and then calculates the value of the coefficient of attenuation of each elementary zone.
The information obtained from these calculations is expressed by a map of the tomographic sectional plane.
The set of maps constitutes the three-dimensional scanner image of the analysis, which permits longitudinal or transverse sections.
The medical interpretation is thus based on a real internal image of the tissues.
Such images enable the condition of certain bones, as well as the condition of the brain, to be checked in order to detect a tumour or other anomaly.
The investigations are preceded or completed by other investigations, for example ultrasound echography or magnetic resonance imaging.
Scanning and the methods that it has introduced remain an essential tool of medical investigation.
At the start, a series of angular displacements of the order of 3° were carried out, repeated some hundred times.
The improvements that have been introduced since then enable a plurality of beams to be combined with detection strips of a sufficient length so as to multiply the number of measurements made at any one time thanks to multiple detectors.
In the fifth generation scanners detector strips are used perpendicular to the sectional plane in order to prevent any shift or displacement.
The image that is obtained is the result of a stepwise process:
obtaining values of the attenuations for each projection;
calculations of the values of a profile;
matrix representation of each sectional plane;
conversion of each representation by means of a specific map;
establishment of a spatial cartographic system.
Nowadays volumes of each elementary zone of the order of mm3 are obtained. However, this is far from the microscopic scale since the number of living cells is of the order of 1 billion per mm3.
An example of such a scanner is described in the document U.S. Pat. No. 5,241,471. According to this document, 256 radiological scannings of the body are carried out by turning the source and the detector 256 times around the body perpendicularly to the sectional plane whose image it is desired to produce. The detector comprises 256 cells, and a variation in intensity of the X-rays between the source and the detector is measured for each scanning, along p=2562 scanning profiles. By means of a suitably programmed computer, the p=2562 values associated with the p=2562 points of the sectional plane are calculated. This calculation is normally performed by an algebraic reconstruction technique (ART). However, the improvement that is advocated here consists in treating first of all only a sub-assembly of the p=2562 scanning profiles, for example a quarter, i.e. 64×256 scanning profiles. These profiles are obtained by averaging by four the p scanning profiles that are actually carried out, or by selecting one in four of these p profiles. The algebraic reconstruction technique is then applied to the values calculated from the sub-group of scanning profiles, to arrive at an image representative of the 2562 points of the sectional plane, in other words the highest possible definition of the image according to the detector that is used.